When a gas laser operates, the electrical discharges between the main electrodes and from any preionization pins that may also be employed cause the erosion of very fine particles of metal from these electrodes and/or pins. The dust that results consists of these particles of metal or of compounds that the metal may have formed with a gas in the laser vessel. In many lasers the gaseous charge is actually a mixture of different gases. However, for the present purposes the charge in the laser vessel, whether a single gas or a mixture of gases, will be referred to simply as "the lasing gas" or just "the gas". If too much of this dust is allowed to accumulate and to circulate in the gas, it can impair the efficiency of the laser operation. It also has a tendency to collect on the optics (mirrors, or windows if external mirrors are used) at each end of the laser cavity, which reduces the performance of these optics.
While this dust problem may be present to some degree in any gas laser, it is particularly acute in an excimer laser. For this reason, although the present invention is applicable to use in any gas laser that experiences the generation of an unacceptable level of dust, it is of primary importance when used with an excimer laser, and hence the examples given below will relate to such latter use.